What is the Ring of Fire? The Pacific's belt of volcanoes

The Ring of Fire is a belt about 40,000 km long around the edges of the Pacific Ocean where most of the world's earthquakes and volcanic eruptions happen. It runs up the west coast of the Americas, across Alaska and down through Japan, the Philippines and Indonesia to New Zealand. It exists because, almost all the way around, the floor of the Pacific is sinking beneath the plates that surround it, a process called subduction.

In short

• About three quarters of Earth's volcanoes and about 90% of its earthquakes lie along the Ring of Fire, according to National Geographic.
• It isn't one structure but a chain of separate subduction zones, each where an ocean plate dives under its neighbor.
• It has gaps, most notably in California, where plates slide past each other instead, and it is shaped more like a horseshoe than a ring.

Where is the Ring of Fire?

Start at the southern tip of South America and follow the coast north: the Andes, Central America and Mexico, a gap through California, then the Cascade Range of Oregon and Washington, the coast of Alaska and the long arc of the Aleutian Islands. From there the belt swings west and south: Kamchatka, the Kuril Islands, Japan, the Mariana Islands, the Philippines, Indonesia, Papua New Guinea, Tonga and New Zealand.

Its length, about 40,000 km according to National Geographic, is about the distance around the Earth at the equator. Its edges are a matter of definition. Some geologists include the Antarctic Peninsula and others don't, and western Indonesia, whose volcanoes face the Indian Ocean rather than the Pacific, is often counted although it is not strictly on the Pacific's rim. That is why counts of how many volcanoes it holds vary, from "more than 450" up.

Why are there so many volcanoes and earthquakes there?

The Pacific Ocean floor is old, cold and heavy, and almost everywhere around its rim it is sinking under lighter plates. One process, subduction, causes nearly everything the Ring of Fire is known for. As the note on plate tectonics explains, subduction does four things:

1. It digs trenches. Where the ocean plate bends down, the seafloor drops into a trench, including the deepest place on Earth, the Mariana Trench.
2. It makes the largest earthquakes. The two plates lock together along a huge sloping fault, called a megathrust, that can be hundreds of kilometers long. When it slips, it can produce earthquakes of magnitude 9, the biggest the planet makes, and lift the seafloor to send out tsunamis.
3. It makes deep earthquakes. The sinking slab keeps cracking as it goes down, so earthquakes occur along it to depths of about 700 km, deeper than anywhere else.
4. It feeds volcanoes. The slab carries water down. At about 100 km depth the water is driven off into the hot mantle above, lowers its melting point and makes magma, which rises to form a line of volcanoes parallel to the trench. Their sticky, gas-rich magma is why so many of them explode, as the note on how volcanoes form explains.

A rough calculation shows how much ocean floor this swallows. If the plates sink along about 40,000 km of rim at an average of about 5 cm a year, then each year a strip 40,000,000 m long and 0.05 m wide goes down: 40,000,000 × 0.05 = 2,000,000 m², or about 2 km². Over a million years, that is about 2 million km² of seafloor, an area the size of Mexico. It is why the Pacific floor, unlike the continents, is nowhere much older than about 200 million years: it is constantly being recycled. The real rate varies widely from place to place, so this is an order of magnitude, not a measurement.

The Ring of Fire, segment by segment

Each section has its own pair of plates and its own history. These are landmark events, one or two for each.

Segment	What sinks under what	Landmark event
Chile and Peru, the southern and central Andes	Nazca Plate under South American Plate	22 May 1960, Valdivia, Chile: magnitude 9.5, the largest earthquake ever recorded
Colombia, the northern Andes	Nazca Plate under South American Plate	13 November 1985, Nevado del Ruiz: a small eruption melted summit ice, and a mudflow buried the town of Armero, killing more than 20,000 people
Mexico and Central America	Cocos Plate under North American and Caribbean Plates	19 September 1985, Mexico City: a magnitude 8.0 earthquake on the coast, 350 km away, shook the soft lakebed under the city
California	No subduction: the Pacific Plate slides past the North American Plate on the San Andreas Fault	18 April 1906, San Francisco: a gap in the volcanoes, but not in the earthquakes
The Cascades, Oregon to British Columbia	Juan de Fuca Plate under North American Plate	18 May 1980, Mount St. Helens erupts; and 26 January 1700, a magnitude 8.7–9.2 earthquake dated from the tsunami it sent to Japan
Alaska and the Aleutians	Pacific Plate under North American Plate	27 March 1964, the Great Alaska earthquake, magnitude 9.2; June 1912, Novarupta, the largest eruption of the 20th century by volume
Kamchatka and Japan	Pacific Plate under the Okhotsk Plate, often counted as part of the North American Plate	11 March 2011, the Tōhoku earthquake, magnitude 9.0–9.1, and its tsunami
The Marianas	Pacific Plate under Philippine Sea Plate	The Challenger Deep, nearly 11,000 m down
The Philippines	Plates converging from both sides	15 June 1991, Pinatubo: the second-largest eruption of the 20th century, after which global temperatures dipped for about two years
Indonesia	Indo-Australian Plate under Sunda Plate, on the Indian Ocean side	27 August 1883, Krakatoa; 1815, Tambora, the largest eruption in recorded history
Tonga and New Zealand	Pacific Plate under Indo-Australian Plate, and the reverse	15 January 2022, Hunga Tonga–Hunga Haʻapai: an underwater eruption whose plume rose more than 50 km

The California row is the one that surprises people. The San Andreas is a transform fault, where plates slide sideways, so there is no sinking slab, no water carried down and no volcanoes: the Cascades stop just north of it. It still produces large earthquakes, by the sideways slipping described in the note on what causes earthquakes. The Ring of Fire's single cause has exceptions, and that is the biggest.

Why isn't the Atlantic ringed with fire too?

Because the Atlantic's edges aren't plate boundaries. The Atlantic is still widening from the Mid-Atlantic Ridge down its center, and the continents on either side ride on the same plates as the ocean floor next to them, so the coasts of Europe, Africa and the eastern Americas are quiet by comparison. The Pacific is the opposite: an ocean being closed up at its edges. A few short subduction zones do exist in the Atlantic, in the Caribbean and near the South Sandwich Islands, and they have volcanoes and earthquakes of their own.

Not every danger in the region comes from the ring itself. Hawaii's volcanoes sit in the middle of the Pacific Plate over a hotspot, far from any edge.

What does living on the Ring of Fire mean?

Hundreds of millions of people live along the Ring of Fire, in cities such as Tokyo, Manila, Jakarta, Lima, Santiago, Mexico City, Los Angeles, San Francisco, Seattle and Vancouver. The same processes that endanger them also built the land they live on: the Andes, Japan and Indonesia are made largely of rock brought up by subduction, and the volcanic soils are among the most fertile on Earth.

Living with it has made the region a leader in earthquake science. Japan and Chile design buildings to sway rather than snap; Japan, Mexico and the western United States run earthquake early-warning systems; and tsunami warning centers watch the whole Pacific, because a megathrust earthquake on one side can send waves across the ocean, as Chile's in 1960 did to Hawaii and Japan. None of these systems can say when the next great earthquake will come. They can only shorten the time between its start and the warning.

Check yourself

• Without looking back: what single process explains the Ring of Fire's trenches, great earthquakes and volcanoes?
• Why does California have big earthquakes but no volcanoes along the San Andreas?
• Why is the Pacific ringed by volcanoes when the Atlantic isn't?

Sources

• Ring of Fire — National Geographic Society
• Plate tectonics and people, in This Dynamic Earth — U.S. Geological Survey
• Understanding plate motions, in This Dynamic Earth — U.S. Geological Survey
• 1700 Cascadia earthquake — Wikipedia
• Ring of Fire — Wikipedia